1. Field of the Invention
The invention relates to a packaging film which is transparent to UV light and has a barrier action against water vapour and gases. Within the scope of the invention is also a process and a use of such a packaging film.
2. Background Art
The sterilization of transparent products by radiation with UV light is known. It is used today e.g. for decontaminating drinking water. It is also known that the storage time of natural untreated foodstuffs can be extended if these are radiated with UV light The UV rays have a decontaminating action only on the surface of a foodstuff. An essentially untreated foodstuff is however sterile on the inside, so that the radiation of the surface is sufficient for decontamination purposes.
In order to keep a product sterile, it is appropriate to perform the sterilization of the foodstuff while it is in the packaging; this way it is not possible for the contents subsequently to come into contact with non-sterile surfaces.
The packaging materials that are known today and are used for packaging foodstuffs either have no adequate barrier action against water vapour or oxygen, or are not transparent to UV radiation. Especially when employing very high energy (short wave) UV light, as is supplied e.g. by excimer lamps, the choice of possible packaging materials is very limited. For example, among others, polyethylene-terephthalate (PET) cannot be used because of its absorption edge at 330 to 350 nm, nor polyvinylidenchloride (PVDC). The choice of possible packaging materials that are transparent to UV light is limited therefore to polyamides (PA) and polymers and copolymers of ethylvinyl-alcohol (EVOH). Often, however, it is not possible to achieve adequate barrier values against water vapor. The barrier against gases and aromas is also often not sufficient.
It has already been found that the barrier properties of plastics can be improved in some cases by thin film vacuum deposition of SiO.sub.x or aluminium oxide coatings using methods such as PVD (physical vapor deposition) or by CVD (chemical vapor deposition). However, the processes and coatings used up to now have yielded substantial improvements with respect to the barrier action against water vapor and gases only with PET or the layers known to date are discoloured i.e. yellowish or greyish and consequently have high absorption in the UV range.